1. Field of the Invention
The present invention relates to a method of manufacturing piezoelectric actuator, a piezoelectric actuator, and an ink-jet head which includes the piezoelectric actuator.
2. Description of the Related Art
A piezoelectric actuator which drives an object by deforming a piezoelectric layer by an action of an electric field on the piezoelectric layer has been hitherto known. An actuator disclosed in Japanese Patent Application Laid-open Publication No. 2003-154646 (FIG. 3) which is used in an ink-jet recording apparatus is one of such type. The piezoelectric actuator according to this patent document includes a piezoelectric layer which is formed by lead zirconate titanate (PZT), a plurality of lower electrodes which are provided on an under surface of the piezoelectric layer to correspond to a plurality of pressure chambers, respectively of an ink-jet head, and a plurality of upper electrodes which are provided on a top surface of the piezoelectric layer to correspond to a plurality of pressure chambers respectively. When drive voltage is supplied to the piezoelectric actuator, there is a difference in electric potential of the lower electrode and that of the upper electrode. Due to the difference in the electric potentials, the electric field acts on the piezoelectric layer which is sandwiched between the two electrodes, thereby deforming the piezoelectric layer, and pressure is applied to ink in the pressure chambers via the lower electrodes which also serves as a vibration plate.
This piezoelectric actuator is manufactured as described below. To start with, the upper electrode is formed on an MgO (magnesium oxide) single crystal substrate by sputtering method. Then, a layer of PZT is formed on a surface of the upper electrode by the sputtering method and the piezoelectric layer is formed by performing a heat treatment on the PZT layer. Furthermore, the lower electrode is formed on the surface of the piezoelectric layer by using the sputter method.
However, in the piezoelectric actuator disclosed in this patent document, there is a substantial difference in a coefficient of thermal expansion between a conductive material of the electrode and PZT of the piezoelectric layer which is in contact with the electrode. As a result, there is a substantial difference in amount of deformation between the PZT and the electrode during a process of forming the piezoelectric layer by means of a heat treatment of PZT. Therefore, interface stress between the piezoelectric layer and the electrode increases, so that the piezoelectric layer tends to exfoliate easily from the electrode. This results in deterioration of durability of the piezoelectric actuator and there is a decline in the yield at the manufacturing stage.